1. Field
The described embodiments relate to electronic devices. More specifically, the described embodiments relate to wireless communication between electronic devices.
2. Related Art
Many modern electronic devices include a networking subsystem that is used to wirelessly communicate with other electronic devices. For example, these electronic devices can include a networking subsystem with a cellular network interface (UMTS, LTE, etc.), a wireless local area network interface (e.g., a wireless network such as described in the Institute of Electrical and Electronics Engineers (IEEE) standards 802.11), and/or another type of wireless interface. In some of these electronic devices, the network interfaces can be used to detect other electronic devices as part of a process for enabling communication between the electronic devices. For example, one electronic device can broadcast an advertising frame, and another electronic device can monitor for the advertising frame to detect the electronic device.
Many of these electronic devices, particularly battery-powered electronic devices, have low power-consumption targets, and so the power that can be allocated for detecting other devices can be minimal. For example, for some devices, a power consumption target for detecting other devices can be in the range of 10 mW/hour. Because the electronic devices require minimal power consumption while detecting other devices, the use of the radios (which typically consumes the largest portion of the allocated power) to detect other devices can be very limited. For example, given the 10 mW/hour target, as little as a few percent of possible radio time (e.g., around 3% of radio time) can be available for detecting other electronic devices.
In order to discover other electronic devices that are infrequently monitoring for advertising frames (e.g., when another electronic device is switching in and out of a reduced-power “sleep” state and hence may be monitoring for advertising frames infrequently), a passive scan procedure can be used. Differently than an active transmit procedure that consists of transmitting advertising frames with broadcast probe request frames at arbitrary times from an electronic device, and thus requesting an answer from the other electronic devices and requiring the other electronic device to be monitoring for advertising frames, the passive scan procedure consists in listening on a given wireless channel with a listening time, i.e., a “dwell time,” that is long enough to catch a advertising frame from the other electronic device. Existing implementations of passive scanning use a 110-120 ms dwell time in order to detect periodic advertising frames spaced at a constant 100-104 ms.
However, there are two problems with existing implementations that use the constant periods for passive scans that become particularly apparent when multiple devices are attempting to discover one another. A larger number of devices means that many devices in range of each other may be advertising, leading to relatively long discovery times due to contention for channels and the need for individual discovery of multiple other devices. This can lead to a power-consumption problem because, even using the radios as sparingly as possible, the power consumption required for broadcasting advertising frames to be detected using passive scans and performing passive scans is high in comparison with power targets for many battery-powered electronic devices.